1. Field of the Invention
This invention relates to methods and systems for making plastic cellular parts and thermoplastic composite articles utilizing such plastic cellular parts.
2. Background Art
One type of recyclable, lightweight, high-strength, composite material or article comprises a “honeycombed” cellular core positioned between two thermoplastic skins reinforced with glass and polypropylene. Polypropylene is highly regarded for its heat and chemical resistance as well as for its ability to withstand wear and tear. The thermoplastic skins, tough and meltable for reuse, have a degree of elasticity between 5 and 20GPa, depending on fiber content and orientation. The composite article typically ranges in thickness between 5 and 30 mm. Its weight ranges from 1700 to 6000 g/m2, depending on skin and core materials.
In contrast to more conventional thermoset resin composites, thermoplastics used in the composite article provide greater robustness due to their tougher matrix. They also offer enhanced formability and functional integration, consist of less expensive raw materials and can be processed faster. Also, living hinges (i.e., U.S. published application 2005/0189674) and deep draw shapes (i.e., U.S. Pat. Nos. 6,682,675; 6,682,676; 6,790,026; and 6,981,863) can be made with the composite article while maintaining structural integrity.
Via thermocompression, production is a one-step process that takes approximately one minute (i.e., U.S. Pat. Nos. 6,050,630 and 6,537,413). Simultaneous exposure to heat and pressure changes the “sandwich” to a thermoplastic composite, yielding high-strength-to-weight and high-stiffness-to-weight properties as well as a finished product that is highly resistant to heat, impact and corrosion.
Applications for such thermoplastic composite materials or articles include pallets (i.e., U.S. Pat. Nos. 6,655,299; 6,748,876; and 6,823,803), vehicle load floors (i.e., U.S. Pat. No. 6,843,525), under-engine fairings (U.S. Pat. No. 6,435,577), inner roof panels (U.S. Pat. No. 6,890,025), trunk panels, backrests, aerodynamic skid plates, spare wheel pans, and front and rear vehicle bumpers.
One way to make the interior plastic cellular core or honeycomb part is to make the core from a plurality of small co-extruded tubes bonded to each other along their sides (i.e., U.S. Pat. No. 5,683,782). The small tubes have a base body made of a thermoplastic and which, at least on the outside, preferably on the outside and on the inside, carries a coating made of an adhesively-active thermoplastic material. As a result of this coating, a bonding of the small honeycomb tubes to each other as well as to a cover layer is possible.
Another way to make a plastic cellular or honeycomb part is to make the fiber-reinforced thermoplastic honeycomb in a continuous manner one half cell at a time by laying down a corrugated web of thermoplastic, with and without fiber-reinforcement atop a honeycomb, selectively fusing the node-antinode demes and repeating the process until a honeycomb of the desired depth is prepared (i.e., U.S. Pat. No. 5,139,596).
Yet still another way to make a plastic cellular or honeycomb part is to injection mold the honeycomb part in plastic (U.S. published application Nos. 2002/0043747 and 2004/0241383).
A common problem in the molding of plastic components is that the solidified plastic component often sticks to the component-defining surfaces of the mold. As a result, the mold cycle times are unnecessarily extended. Also, the article-defining surface as well as the resin flow path within the mold must be cleaned and/or lubricated on a periodic basis to ensure that subsequently molded plastic components are not contaminated with plastic that was previously stuck within the mold. This problem is especially acute where color of the plastic component has been changed.
One way of reducing this sticking problem is to utilize a mold release spray or lubricant wherein the spray is deposited on the surface layers which define the article-defining cavity. However, the use of such a spray is not only time-consuming but expensive.
Another way of reducing the sticking within the article-defining cavity is to incorporate release agents within the plastic itself. However, the use of such release agents present other problems including the expense of incorporating such release agents within the plastic material.
Another way of reducing the sticking problem is to use increased molding pressures. However, here again, the use of increased molding pressures adds even more problems to the molding process.
U.S. Pat. No. 6,686,007 discloses a plastic injection mold having at least one self-lubricating surface layer which provides at least one substantially non-stick surface.